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Andromeda X: Andromeda's Newest Satellite Galaxy

By Ken Croswell

January 29, 2006

Image by Daniel Zucker, using the Nordic Optical Telescope.

The discovery of a new galaxy orbiting Andromeda gives a
boost to the leading theory of galaxy formation, say astronomers
in Europe and the United States. The new galaxy, named Andromeda
X (pronounced "Andromeda ten"), is the dimmest satellite galaxy
ever seen around Andromeda.

Theorists believe giant galaxies such as Andromeda and the
Milky Way--the two largest galaxies in the Local Group--formed
when smaller galaxies smashed together. Trouble is, this theory
predicts the existence of lots of small galaxies, far more than
observers actually see. In particular, the theory says Andromeda
and the Milky Way should have hundreds of satellite galaxies.

Now Daniel Zucker of the Max Planck Institute for Astronomy
in Heidelberg, Germany, and his colleagues have discovered a
satellite of Andromeda that is so faint it suggests many others
exist unseen, perhaps resolving the discrepancy. The astronomers
spotted the galaxy while examining data from the Sloan Digital
Sky Survey. Then they observed the galaxy with two telescopes in
the Canary Islands, the 4.2-meter William Herschel Telescope and
the 2.5-meter Nordic Optical Telescope.

Andromeda X--the "X" is the Roman numeral ten--is a dwarf
spheroidal galaxy, a diffuse collection of stars spread out from
one another. Its absolute magnitude is a meager -8.1, making the
entire galaxy only slightly more luminous than Deneb in Cygnus
(absolute magnitude -7.6) and Rigel in Orion (absolute
magnitude -6.8).

In 2004, Zucker and his colleagues reported the discovery of
another Andromeda satellite, Andromeda IX, whose absolute
magnitude is -8.3. At the time, it was the least luminous galaxy
known. However, last year Beth Willman of New York University
and her colleagues announced the discovery of the Ursa Major
dwarf, a satellite of the Milky Way, with an absolute magnitude
of roughly -6.75--the faintest galaxy ever seen.

But Martin Rees of Cambridge University says these discoveries may not matter so much. "I don't believe there's a severe discrepancy between the current dark matter models and what we observe," he says. Rees thinks the conflict between the predicted and observed numbers of dwarf satellites can be resolved if each known dwarf has more dark matter, and thus more mass, than current observations suggest. "If you associate the observed dwarf galaxies of our Galaxy with 109-solar-mass halos rather than 108-solar-mass halos, then the numbers work out fine," says Rees.

Astronomers estimate the dark-matter supply in a dwarf galaxy by observing its stars' speeds: the faster the stars move relative to one another, the more dark matter the galaxy must have to hold itself together. But Rees says these observations underestimate a dwarf galaxy's mass, because the stars reside near the galaxy's center, while most of the dark matter is in a halo on the galaxy's outskirts.

Zucker and his colleagues find that the brightest stars in Andromeda X are red giants, aging stars that evolve from stars like the Sun. However, the stars in Andromeda X have only about 1 percent of the solar abundance of iron and other heavy elements.

The red giants' brightness indicates that Andromeda X is slightly closer to us than the Andromeda Galaxy is. Whereas Andromeda's distance from Earth is
2.5 million light-years, Zucker's team estimates Andromeda X is
2.2 to 2.4 million light-years from us. The small galaxy is
280,000 to 450,000 light-years from Andromeda's center. Its
angular separation from the center of the Andromeda Galaxy is 5.5
degrees, about the same as the separation between the two
"pointer stars" in the Big Dipper's bowl.

Andromeda X is the fifteenth known satellite of the
Andromeda Galaxy. Andromeda's brightest satellite, the spiral
galaxy M33, emits about 20 percent as much light as the Milky
Way. By contrast, Andromeda X gives off only 0.001 percent as
much light as the Milky Way. It would take 100,000 galaxies like
Andromeda X to equal the radiance of our Galaxy.

In work submitted to Astrophysical Journal Letters, Zucker
and his colleagues predict that many other dim galaxies likely
orbit Andromeda and the Milky Way: "The discoveries of Andromeda
IX and Andromeda X suggest that such extremely faint satellites
may be plentiful in the Local Group."